Project Summary A functional decline in smell, which is experienced by nearly three-quarters of Americans by age 80, can endanger nutritional status, physical safety, and negatively impact quality of life. Unfortunately, there are currently no effective treatments to reverse age-related hyposmia. This sensory deficit can be attributed to pathologically deteriorated regions within aged human olfactory epithelium (OE) that are void of olfactory sensory neurons. Aneuronal regions persist because horizontal basal cells (HBCs), the OE's reserve resident stem cell population that is capable of giving rise to all OE cell types, remain frustratingly dormant. Activating multipotent HBCs that endure within aged human OE, therefore, represents a potentially powerful approach to reverse smell loss. Our laboratory has previously demonstrated that conditional knock out (cKO) of the Notch receptor, Notch1, in HBCs leads to decreased HBC expression of the transcription factor (TF) and master regulator of HBC status, p63, and precipitates spontaneous HBC activation. However, to deftly manipulate the only signaling pathway currently known to regulate the molecular and functional status of HBCs, we require a deeper understanding of Notch signaling and its role in specifying HBC status. This need is due to the inherent complexities of the Notch signaling pathway. One aspect of this intricacy includes the ability for a single Notch receptor type to interact with multiple ligands, each of which can differentially impact cellular differentiation. In the OE, Notch1 expressed by HBCs can potentially interact with several Notch ligands, including Dll1 that is also expressed by HBCs and Jagged1 localized to glial-like Sustentacular cells. As my data demonstrates that HBC-expressed Dll1 and Notch1 do indeed interact, Aim 1 will establish Dll1's effect on the functional and molecular identity of dormant HBCs (dHBCs) within uninjured OE by (i) quantifying Hes1, a Notch signaling- dependent TF, and p63 expression following Dll1 cKO in primary mouse HBC cultures. I will also (ii) utilize fluorescence activated cell sorting to quantify in vivo HBC expression of Hes1 and p63 following HBC cKO of Dll1. To probe the longer-term functional role of Dll1 in HBCs, I will (iii) quantify the number and identity of HBC-derived progeny following HBC cKO of Dll1. Because the functional output of Notch ligand-receptor interactions can be dynamically regulated during tissue morphogenesis, Aim 2 will elucidate the influence that Dll1 exerts on activated HBCs (aHBCs) in a model of HBC-mediated OE regeneration elicited by direct OE injury. To do so, I will (i) perform flow cytometry to correlate, with cellular resolution, the relationship between HBC activity status and Dll1 expression. I will also (ii) interrogate the functional and molecular impact that Dll1 has on regeneration by overexpressing Dll1 in HBCs via viral transduction in the immediate aftermath of OE injury. In line with NIDCD's mission, this project and its multidisciplinary approach will aid the design of medical treatments that therapeutically activate HBCs and controllably regenerate the OE to reverse age-related hyposmia. As a result, I will be well prepared by this fellowship to embark on a career as a physician-scientist.